A cyclotron is a type of circular particle accelerator in which negatively or positively charged particles are accelerated outwards from the centre of the cyclotron along a spiral path up to energies of several MeV. Unless otherwise indicated, the term “cyclotron” is used in the following to refer to isochronous cyclotrons. Cyclotrons are used in various fields, for example in nuclear physics, in medical treatment such as proton-therapy, or in radio-pharmacy. In particular, cyclotrons can be used for producing short-lived positron-emitting isotopes suitable for PET imaging (positron emitting tomography) or for producing gamma-emitting isotopes, for example, Tc99m, for SPECT imaging (single photon emission computed tomography).
A cyclotron generally comprises several elements including an injection system, a radiofrequency (RF) accelerating system for accelerating the charged particles, a magnetic system for guiding the accelerated particles along a precise path, an extraction system for collecting the thus accelerated particles, and a vacuum system for creating and maintaining a vacuum in the cyclotron.
A particle beam constituted of charged ions is introduced into a gap at or near the center of the cyclotron by the injection system with a relatively low initial velocity. As illustrated in FIG. 3, this particle beam is sequentially and repetitively accelerated by the RF accelerating system and guided outwards along a spiral path comprised within the gap by the magnetic field generated by the magnetic system. When the particle beam reaches its target energy, it can be extracted from the cyclotron by the extraction system provided at a point of extraction, PE. This extraction system can comprise, for example, a stripper consisting of a thin sheet of graphite. For example, H− if ions passing through the stripper lose two electrons and become positive. Consequently, the curvature of their path in the magnetic field changes its sign, and the particle beam is thus led out of the cyclotron towards a target. Other extracting systems exist which are well known to the persons skilled in the art.
The magnetic system generates a magnetic field that guides and focuses the beam of charged particles along the spiral path until it is accelerated to its target energy. In the following, the terms “particles”, “charged particles”, and “ions” are used indifferently as synonyms. The magnetic field is generated in the gap defined between two magnet poles by two solenoid coils, 14, wound around these poles. Magnet poles of cyclotrons are often divided into alternating hill sectors and valley sectors distributed around a central axis. The gap between two magnet poles is smaller at the hill sectors and the larger at the valley sectors. A strong magnetic field is thus created in the hill gap portions within the hill sectors and a weaker magnetic field is created in the valley gap portions within the valley sectors. Such azimuthal magnetic field variations provide radial and vertical focusing of the particle beam every time the particle beam reaches a hill gap portion. For this reason, such cyclotrons are sometimes referred to as sector-focusing cyclotrons. In some embodiments, a hill sector has a geometry of a circular sector similar to a slice of cake with a first and second lateral surfaces extending substantially radially towards the central axis, a generally curved peripheral surface, a central surface adjacent to the central axis, and an upper surface defining one side of a hill gap portion. The upper surface is delimited by a first and second lateral edges, a peripheral edge, and a central edge.
In practice, a particle beam has a cross sectional area. An objective of cyclotrons is to produce charged particle beams having a given energy which are as much focused as possible (i.e. having a small cross sectional area). The variations of the magnetic field created by the succession of hill sectors and valley sectors contributes to the focusing of the beam in a similar way as a light beam can be focused by lenses. Upon extraction of the particle beam out of the gap defined between two magnet poles, however, the particle beam crosses boundary regions where the magnetic field loses its homogeneity, which is detrimental to the focusing of the particle beam. This is a particularly sensitive issue because, on the one hand, the particle beam has its highest energy at the point of extraction and, on the other hand, it is more difficult to control the magnetic field at the peripheral edges of the magnet poles where the magnetic field drops rapidly. To enhance the focusing of an extracted particle beam, it has been proposed in the art to modify the geometry of the peripheral edges of hill sectors by forming protrusions to said peripheral edges by addition of gradient correctors. Gradient correctors are relatively small blocks of steel with respect to the size of a hill sector, which are coupled to the peripheral surfaces of the hill sectors. Such gradient correctors allow the modification of the magnetic field near the peripheral edges and thus locally modify the magnetic field near the peripheral edge of a hill sector to improve the focusing of the outgoing particle beam. The use of protruding gradient correctors has, however, several drawbacks. First, the volume of the vacuum chamber hosting the magnet poles must be increased accordingly, thus requiring more energy and time to pump the gases from the vacuum chamber. Second, the overall weight of the cyclotron is increased because of, on the one hand, the weight of the gradient correctors themselves and, on the other hand, the increased overall size of the outer walls of the vacuum chamber and, consequently, the size of the flux return yoke; both contributing to a substantial increase of the cyclotron weight. Third, the position of the protruding gradient correctors is essential; small deviations of position may yield large variations of the magnetic field. Gradient correctors must be fixed manually by a skilled artisan at precisely the same position of the peripheral surface of all the hill sectors. This is of course, a critical and expensive operation. Fourth, these protruding gradient correctors have the effect of deviating the magnetic field outwards, which pulls outwards the path of the particle beam towards the peripheral edge of a hill gap portion between a pair of opposed hill sectors where the magnetic field loses its homogeneity. This shift also leads to a loss of useful magnetic field and thus requires an increase of the coil current in order to compensate this loss. It is therefore more difficult and expensive to control the properties of the extracted particle beam.
There therefore remains a need in the art to provide an isochronous sector-focused cyclotron allowing the extraction of a more focused and more predictable particle beam in an efficient and cost effective manner.